In construction applications, mortars may be prepared with cement, sand, and organic polymer. To reduce shipping costs, the polymer can be shipped and added in dry form as a redispersible polymer powder. Redispersible polymer powders are used as binders to improve the adhesion of cementicious adhesive formulations. The redispersible polymer powders should provide high bond strength to both surfaces to be joined, such as to both concrete and expanded polystyrene boards (EPS), which are used as insulation material in external thermal insulation systems (ETICS), and to tiles and flooring or wall board in cement based tile adhesives. Another key performance requirement besides adhesion in applications such as EPS board in ETICS is to provide impact resistance to physical stress. However, generally high bond strength is in conflict with the high flexibility required to give good impact resistance.
International Patent Publication No. WO 99/32541 to Kuehn et al, and its U.S. National Stage patent family member U.S. Pat. No. 6,451,900, discloses a blend of two polymers, where Polymer I is a film forming polymer at room temperature and Polymer II is water insoluble at neutral pH and contains carboxylic groups. Polymer II works as a protective shell. It has a much smaller particle size (by a factor of about 5 to 150) and a higher Tg. However, Polymer II does not contribute to the film and cannot be considered as a binder.
Mortars formulated with the styrene butadiene redispersible polymer powders (SB RDPs) of the present invention exhibit both an unexpectedly high bond strength and high impact resistance in cementicious adhesive formulations, such as those employed with expanded polystyrene boards (EPS), which are used as insulation material in external thermal insulation systems (ETICS), and cement based tile adhesives (CBTAs). Accordingly, the present invention solves the problem of having high bond strength and high impact resistance at the same time in cementicious adhesive formulations by use of a carboxylated styrene butadiene redispersible polymer powder prepared from a blend of different carboxylated SB latexes or polymers having different glass transition temperatures (Tgs) in critical particle size ratios.